The human cytomegalovirus (HCMV) protease is a member of the herpes protease family. All the herpes proteases are highly homologous and essential for the maturation of herpes virus. Crystal structure analysis of the inhibitor complex of HCMV protease will: (a) broaden our understanding the inhibition mechanism of HCMV protease and other herpes proteases as well; (b) provide critical structure information for the design of more potent inhibitors.